Linear butenes (1−C4=and/or 2−C4=) are important feedstocks in a number of commercial processes. However, such feedstocks are often also required to contain low levels of both butadiene and isobutylene (iC4=) impurities. For example, 1-butene (1−C4=) can be used in the production of linear low density polyethylene (LLDPE), but the feedstock must contain less than 50 ppm of butadiene and less than 0.3˜1.0 wt % isobutylene. Similarly, linear butenes can be oligomerized to produce near linear alpha olefins (NLAOs) useful in the production of surfactants, but again there are severe restraints on the levels of butadiene and isobutylene in the feedstock.
A variety of processes are available for producing C4 olefinic streams including, for example, steam cracking, fluid catalytic cracking, catalytic naphtha cracking and the conversion of methanol to olefins (MTO). However, in addition to linear butenes, the C4 olefinic streams resulting from these processes typically contain significant quantities of butadiene isobutylene and saturated C4 hydrocarbons (n-butane and iso-butane). Accordingly, before they can be used in processes such as the production of LLDPE, these olefinic streams must be treated to reduce the butadiene and isobutylene impurities to acceptable levels.
Separation of butadiene or isobutylene from 1-butene by fractionation is difficult due to the close boiling points of the two impurities to 1-butene. Such fractionation processes therefore require large distillation towers and can be prohibitively expensive. A practiced option is extractive distillation with a selective solvent. However, although butadiene can be effectively removed from C4 olefins by this technique, similar technology is unavailable for removal of isobutylene. Moreover, butadiene extractive distillation requires additional equipment for solvent separation and recovery.
Selective hydrogenation processes are capable of removing butadiene from C4 olefinic streams by conversion of the butadiene to butenes, butane, or dimerized (C8) or trimerized (C12) products. Thus, for example, U.S. Pat. No. 5,227,553 discloses a process for the selective hydrogenation of butadiene to butenes in the liquid phase or trickle phase in contact with a fixed-bed of supported noble metal catalyst, wherein a C4 stream having a butadiene content of from 20 to 80% by weight is hydrogenated in a cascade of two reaction zones such that the hydrogenation product from the first reaction zone has a butadiene content of from 0.1 to 20% by weight and the hydrogenation product from the second reaction zone has a butadiene content of from 0.005 to 1% by weight.
In addition, U.S. Pat. No. 6,169,218 discloses a process for the removal of diolefins and acetylenic compounds in an olefin rich aliphatic hydrocarbon stream by selective hydrogenation at a temperature 40 to 300° F. and low hydrogen partial pressure in the range of about 0.1 psia to less than 70 psia in a distillation column reactor.
However, although selective hydrogenation processes are effective in removing butadiene without significant loss of linear butenes in the feed, they cannot remove isobutylene in a similar manner. Thus the order of hydrogenation activity of the olefinic C4 species is as follows: butadiene >1−C4=>iC4=. Thus, isobutylene cannot be hydrogenated without first hydrogenating the desired 1-butene.
One known process for separating isobutylene from linear butenes is via the production of methyl tert-butyl ether (MTBE). In this process, methanol is reacted selectively with isobutylene in a C4 olefinic stream using an acidic catalyst to produce MTBE with little loss of linear butene product. Although effective, this method of separation has some disadvantages. Thus, butadiene cannot be removed under the same conditions and the formation of MTBE is not 100% selective so that additional oxygenates are also produced. Moreover, extensive recovery equipment is needed to remove the oxygenates and recover any unreacted methanol from the product stream. In addition, there is the potential for future restrictions on the use of MTBE, which could make the production of MTBE unattractive.
There is therefore a need for a process which is effective to selectively remove both isobutylene and butadiene from C4 olefinic streams without the need for extensive recovery equipment.
U.S. Pat. No. 4,454,367 discloses a process for removing isobutene in 1-butene without loss of the 1-butene by selectively oligomerizing the isobutene over a high silica mordenite catalyst.
U.S. Pat. No. 5,955,640 discloses an integrated process for the production of butene-1 from a C4 hydrocarbon stream in which the hydrocarbon stream is initially fed to a selective hydrogenation section to remove butadiene and acetylenic compounds. The hydrogenated stream is then fed to a distillation section to separate a butene-1 product fraction and then to a molecular sieve separation section to remove butenes. The remaining hydrocarbon stream (composed of butene-1 and butenes-2) is then fed to a double bond isomerization section to convert the cis and trans butenes-2 to butene-1, whereafter the isomerization effluent is mixed with fresh feed and recycled back to the selective hydrogenation section.
U.S. Pat. No. 6,215,036 discloses a process for producing isobutene from a C4 olefinic hydrocarbon cut containing isobutene as well as but-1-ene and but-2-enes. The process comprises passing the hydrocarbon cut into a distillation zone associated with a hydroisomerization reaction zone, wherein the hydroisomerization zone is at least partly external to the distillation zone. The hydroisomerization zone selectively converts the but-1-ene into but-2-enes, which can be separated from the isobutene in the distillation zone.
International Patent Publication Number 01/46095 discloses a process for producing trimethylpentenes by selectively dimerizing isobutene in a C4 olefinic feedstock containing isobutene and n-butenes over a zeolite beta catalyst.
U.S. Patent Application Publication No. 2002/0002316 discloses a process for the production of high octane number hydrocarbons by the selective dimerization of isobutene in a hydrocarbon cut having a linear olefin/isobutene ratio >3 over an acid catalyst, such as phosphoric acid, cationic exchange acid resins, liquid acids such as H2SO4, sulfonic acid derivatives, silica-aluminas, mixed oxides, zeolites, and fluorinated and chlorinated aluminas.
U.S. Patent Application Publication No. 2002/0128528 discloses a process for selectively removing acetylene compounds from the effluent of a steam cracker comprising passing the effluent into a distillation zone associated with a hydrogenation reaction zone external to the distillation zone. The acetylene compounds are converted to olefins and oligomers in the hydrogenation reaction zone and the oligomers are withdrawn as heavies from the distillation zone.